The present invention relates to a structure and method for replacing electrical nets (point to point connections within a circuit board). More particularly, the present invention relates to a structure and method capable of replacing electrical nets in two sided component surface mount carriers as well as single sided board assemblies. The present invention can be employed on traditional xe2x80x9cdogbonexe2x80x9d BGA sites as well as the newer denser BGA patterns where traces extend in different patterns from the pad to the via.
Electrical components are commonly mounted on circuit panel structures such as circuit boards. Circuit panels ordinarily include a generally flat sheet of a dielectric material with electrical conductors disposed on a major, flat surface of the panel or on both major surfaces. The conductors are commonly formed from metallic materials such as copper and serve to interconnect the electrical components mounted to the board. Where the conductors are disposed on both major surfaces of the panel, the panel may have additional conductors extending through the dielectric layer so as to interconnect the conductors on opposite surfaces. Multilayer circuit board assemblies have been made heretofore which incorporate plural, stacked circuit boards with additional layers of dielectric materials separating the conductors on mutually facing surfaces of adjacent boards in the stack. These multi-layer assemblies ordinarily incorporate interconnections extending between the conductors on the various circuit boards in the stack as necessary to provide the required electrical interconnections.
Point to point electrical connections within the circuit board are commonly referred to as nets. However, due to current manufacturing and process limitations in the construction of high layer count, thick, large and correspondingly expensive printed circuit boards, there is a significant yield impact due to defective nets (i.e. nets that are open or shorted).
The present practice to improve the yield in the manufacture of these types of boards has been to delete the defective nets from the board and replace them with discrete wires running on the surface. The discrete wiring is typically added after assembly of the components to the board during the assembly process. The repair of a net that is terminated at a ball grid array (BGA) site presents a hurdle to the repair wiring process in that the interconnect to the device is not readily accessible.
The problem of repair for single sided surface mount technology (SMT) boards is addressed in U.S. Pat. Nos. 5,809,641 and 6,018,866. This method, however, suffers from limitations of use for single sided SMT assemblies with BGA""s that have dogbone connections immediately adjacent to the deleted via. A unique pin is required for varying board thickness and the protrusion of the pin on the bottom side precludes the ability to screen solder paste on the bottom side and thus two sided assembly.
With respect to two sided SMT assemblies with BGA""s located at the end of a defective net, repair has been carried out on the BGA side of the board by running a fine gauge wire (34 ga magnet wire) from BGA pad to BGA pad after having left the affected BGAs off during the initial SMT assembly. The wires were soldered to the BGA pads and glued down along the topside board surface to hold the wire in place during subsequent BGA attachment using a BGA rework tool. The disadvantage of this method was the need to leave off the BGAs during initial assembly and the need to run the fine wire between the BGA pads to escape from the array. This wire was exposed during the rework process to damage and shorting to adjacent BGA interconnects (balls).
In order to reduce the chance of damaging the wire or shorting the adjacent BGA balls, the above method has been modified wherein a fine gauge wire is fed up through the drilled out via. The insulation is stripped from the end of the wire, the end is flattened and soldered to the BGA pad that is associated with the deleted net, an epoxy is then added around and into the via to maintain the wire in place and over and around the wire near the BGA pad to provide a solder dam. This modified method provides improvement over prior practice but still requires that the BGAs be left off during the initial assembly.
Having to leave the BGAs off during initial assembly is a quality and logistics detractor in the assembly of two sided SMT assemblies. Each board, due to its unique defects, requires manual intervention at the automated placement tool to leave off the affected BGAs. It then requires additional steps to site dress, place and reflow the BGA. This operation adds significantly to the cost and cycle time of the assembly.
The present invention addresses prior art problems discussed above. More particularly, the present invention relates to repairing two sided component surface mount carriers as well as single sided board assemblies. According to the present invention, an electrically conductive element is bonded in a deleted (e.g. drilled out) via with a head portion of the electrically conductive element connected to at least one circuit feature on the bottom of the board and the other end of the electrically conductive element electrically connected to a BGA pad or surface trace.
The present invention relates to a structure comprising:
a circuitized laminate with electrical nets having plated through holes;
a top side having circuitized features and a bottom side having circuitized features;
at least one through hole without plating and having insulation along the sidewalls of the through hole;
an electrically conductive element located in the through hole.
The electrically conductive element has a diameter that is less than the inside diameter of the deleted insulated plated through hole such that it slips freely in the deleted insulated plated through hole. The electrically conductive element also has a head, which contacts the bottom side of the laminate. A discrete wire can be attached to the electrically conductive element head. The other end of the electrically conductive element matches the laminated thickness and is electrically connected to circuitized features on the top side of the laminate.
The present invention also relates to a method of replacing an electrical net in a circuitized laminate structure. In particular, the method comprises providing a structure comprising:
a circuitized laminate with electrical nets having plated through holes;
a top side having circuitized features and a bottom side having circuitized features;
locating a through hole electrically connected to an electrical net;
removing plating in said through hole;
providing an electrically conductive element having a first section of a length which matches or exceeds the laminate thickness and of smaller diameter than the through hole, and a second headed section of greater diameter than the through hole;
feeding the first section of the electrically conductive element through the through hole until the head contacts the bottom side of said laminate;
providing insulation in the through hole as a barrier between the through hole and electrically conductive element;
electrically connecting the electrically conductive element to said at least one circuit feature on said top side of said laminate; and
connecting the headed section of the electrically conductive element to said at least one circuit feature on said bottom side of said laminate.
The present invention also relates to a structure obtained by the above process.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.
FIG. 1 is a schematic diagram of a structure during the initial states of the method of the present invention.
FIG. 2 is a schematic diagram of structure employing the present invention.